The present invention generally relates to semiconductor devices and more particularly to a semiconductor photodetection device using a compound semiconductor material.
Semiconductor photodetection devices are used in various technical fields such as optical telecommunication. Particularly, the devices that are formed on the compound semiconductor materials such as InP, InGaAs, and the like, play a major role in the advanced optical telecommunication system. Among various semiconductor photodetection devices, the avalanche photodiode (APD) using InGaAs is particularly important for the long distance optical telecommunication trunks because of the excellent sensitivity.
In the avalanche photodiodes, a member called a guard ring is used in correspondence to a surface electrode for sustaining a large bias voltage applied to the surface electrode for causing the avalanche multiplication of the carriers.
FIG. 1 shows a conventional avalanche photodiode.
Referring to FIG. 1, the avalanche photodiode is constructed on a n.sup.+ -type InP substrate 1. More specifically, there is formed an optical absorption layer 2 of n-type InGaAs on the substrate 1, and an intermediate layer 3 of n-type InGaAsP is provided on the optical absorption layer 2. Further, a n-type InP avalanche multiplication layer 4 is provided on the intermediate layer 3, and a n.sup.- -type InP layer 5 having an impurity concentration level different from that of the layer 4 is provided on the avalanche multiplication layer 4. In a part of the InP layer 5, there is provided a p.sup.+ -type InP region 6 forming a window for receiving optical radiation. Thereby, there is formed a p-n junction at an interface between the InP region 6 and the rest of the InP layer 5.
Around the InP region 6, there is formed a guard ring 7 for ensuring that the avalanche photomultiplication occurs at the p-n junction formed between the region 6 and the layer 5, and the entire structure is protected by a silicon nitride passivation film 8 that covers the upper surface of the InP layer 5. Further, in correspondence to the guard ring 7, there is provided an electrode 9 on the passivation film 8 in electrical contact with the guard ring 7. Furthermore, an opposing electrode 10 is provided at a bottom surface of the substrate 1.
In operation, a large bias voltage is applied across the electrode 9 and the electrode 10 such that the p-n junction at interface between the p.sup.+ -type region 6 and the n.sup.- -type InP layer 5 is reverse biased. Upon incidence of optical radiation through the region 6, the photons of the optical radiation are absorbed by the optical absorption layer 2 and thereby the electrons and holes are formed in the layer 2. The holes then migrate into the region 6 and collected by the electrode 9, while the electrons are collected by the electrode 10 after passing through the substrate 1. As there is established a large electric field in the p-n junction between the region 6 and the layer 5, the holes that entered into the p.sup.+ -region 6 are accelerated and thereby the multiplication of the electrons and holes occurs by the avalanche effect.
It should be noted that the guard ring 7 is provided such that the concentration of the electric field at the outer edge of the p.sup.+ -region 6 is avoided and the avalanche multiplication occurs always at the p-n junction formed at the central part of the region 6. In the illustrated example, the impurity concentration level of the n.sup.- -type InP layer 5 is decreased such that the lateral spreading of the depletion region formed at the p-n junction is facilitated. Thereby, the concentration of the electric field due to the curvature of the guard ring 7 is relaxed and a sufficient breakdown voltage, larger than that of the region 6, is obtained for the guard ring 7 with respect to the n.sup.- -type InP layer 5.
FIG. 2 shows another conventional example. In this structure, there is provided a second, shallow guard ring 11 at the outside of the guard ring 7 for further relaxation of the electric field concentration.
In any of the foregoing examples, it is necessary to increase the impurity concentration level of the n-type avalanche multiplication layer 4 in order to achieve an improved response of the avalanche photodiode. Such a requirement of the improved response is particularly acute in the devices that are used in the optical telecommunications field. However, the increase in the impurity concentration level of the layer 4 invites a decrease in the breakdown voltage between the guard ring 7 and the n.sup.- -type InP region 5. For example, when the layer 4 is doped to the impurity concentration level of 5.times.10.sup.16 cm.sup.-3, the breakdown voltage of the guard ring 7 is decreased to about 50-60 volts. This breakdown voltage is substantially equal to or smaller than the breakdown voltage of about 60 volts of the p.sup.+ -type region 6, and thus, the avalanche photodiode does not operate properly.